Compression molding of shaped composite material products involves compressing polymer resin in a mold under an applied pressure. To form an internal cavity within a compression molded part, it is necessary to provide an additional mold core that is separate from the mold, and that is withdrawn in a direction different from the direction of separation of the mold halves. As will be understood, when the internal cavity includes a threaded surface, the additional mold core cannot be simply axially withdrawn because doing so would interfere with the newly-formed threads.
Several approaches for forming threaded bores during injection molding and compression molding have been described. For example, U.S. Pat. No. 5,135,700 to Williams et al. describes a method and apparatus for molding a product having internal threads that includes a threaded mold core and a device for rotating the mold core about the axis of its threads and for axially withdrawing the core from the mold at a rate that is related to the rate of rotation, according to the pitch of the threads. A cam is rotated in synchronism with the device that rotates the core, and a cam follower displaces the core as the cam is rotated. The cam follower is fixed with respect to the mold. The cam and threaded core are positioned on a carrier, along with the device for rotating the core, such that rotation of the cam displaces the carrier away from the mold. The core is inserted into the mold, prior to the molding process, by displacing the carrier toward the mold without rotating the core. In one embodiment, the cam is rotated by a unidirectional motor such that resetting of the cam after the core is withdrawn is accomplished by forward indexing of the cam a small amount to a zero position.
U.S. Pat. No. 6,602,065 to Ingram describes an apparatus for compression molding plastic closures having a peripheral skirt with an internal thread. The apparatus includes a first mold assembly having a male mold core and a second mold assembly having a female mold cavity. At least one of the first and second mold assemblies is moved relative to the other to bring the male mold core into the female mold cavity for compression molding a closure, and then is moved to open the cavity such that the closure is retained on the male mold core. The male mold core is rotated relative to the first mold assembly to unthread the core from within the closure, and thereby strip the closure from the core, without stretching the closure skirt over the core or wiping the internal thread over the external surface of the mold core.
Improvements are generally desired. It is therefore an object of the present invention at least to provide a novel thread forming device for compression molding and apparatus incorporating same.